LibGUI+LibVT: Notify widgets of emoji selection with a callback

Currently, LibGUI modifies the Ctrl+Alt+Space key event to instead
represent the emoji that was selected through EmojiInputDialog. This is
limited to a single code point.

For multiple code point emoji support, individual widgets now set a hook
to be notified of the emoji selection with a UTF-8 encoded string. This
replaces the previous set_accepts_emoji_input() method.

Userland/Libraries/LibGUI/TextEditor.h

@@ -151,6 +151,7 @@ public:
     void delete_from_line_start_to_cursor();
     void select_all();
     void insert_emoji();
+    void set_or_clear_emoji_input_callback();
     void select_current_line();
     virtual void undo();
     virtual void redo();